This invention relates to hydrotreatment of heavy hydrocarbon streams containing asphaltenes. The invention relates more particularly to partial conversion of said asphaltenes to lower boiling, more valuable products and the removal of those asphaltenes which are not readily susceptible to such conversion.
As refiners increase the proportion of heavier, poorer quality crude oil in the feedstock to be processed, the need grows for processes to treat the fractions containing increasingly higher levels of asphaltenes and metals. Asphaltenes are polynuclear aromatic compounds with molecular weights as high as about 10,000 and generally boil above 1,000.degree. F.
It is widely known that asphaltenes are present in petroleum crude oils and other heavy petroleum hydrocarbon streams, such as petroleum hydrocarbon residua, hydrocarbon streams derived from tar sands, and hydrocarbon streams derived from coals. Much of the metals such as nickel, vanadium, and iron commonly found in such hydrocarbon streams are bound in the asphaltene molecules. In various refining operations the asphaltenes and metals cause interstitial plugging of the catalyst beds and reduced catalyst life and various metal deposits on a catalyst tend to poison or deactivate the catalyst. Moreover, the asphaltenes tend to reduce the susceptibility of hydrocarbon streams to desulfurization.
In the petroleum industry, the asphaltene content of distillation heavy residue, commonly termed resid, has long been a problem for economic conversion of the resid into lower boiling, more valuable products. Hydrocracking has been a well-known method of converting higher boiling feedstocks into lower boiling products but hydrocracking has not generally been commercially successful in converting asphaltene-containing resid feedstock. In general, hydrocracking is the treatment of a feedstock at elevated temperature and pressure in an atmosphere of hydrogen and in the presence of a dual functional catalyst having both hydrogenation and cracking activity. Because of the elevated temperature and pressure employed, hydrocracking requires a substantially higher investment than commonly employed thermal coking which achieves limited resid conversion.
To be a competitive alternative process for treating petroleum resid and other heavy hydrocarbon streams containing asphaltenes, a hydrocracking process must successfully convert a substantial portion of the asphaltenes into lower boiling more valuable products in continuous, reliable operation. Such operation has generally proved unattainable because substantial conversion of the asphaltenes together with the material boiling above 1,000.degree. F. to lower boiling products has required very severe operating conditions, resulting not only in asphaltene plugging of catalyst beds but, in addition, plugging in downstream lines and equipment due to remaining unconverted asphaltenes precipitating out of the hydrocrackate effluent. At about 300.degree. F. hydrotreated asphaltenes remaining in hydrocrackate effluent have less than one-fifth the solubility of virgin asphaltenes in hydrocrackate but above about 400.degree. F., hydrotreated asphaltenes have almost the same solubility as virgin asphaltenes. Even so, when economic operation dictates removal of asphaltenes from the heavy hydrocarbon streams, the processes previously proposed for deasphaltene treatment have not been generally competitive with commercial coking operations.
U.S. Pat. No. 2,715,603 (Lanning et al., 1955) discloses a process for removal of asphaltenes from the recycle stream in hydrogenolysis of heavy oil using suspended catalyst; the process disclosed employs a single-stage, severe hydrotreating reaction under very high pressure with the fractionated heavy stream deasphaltened and recycled to extinction and overall the process would produce only a distillate product.
U.S. Pat. No. 3,132,088 (Beuther et al., 1964) discloses a process for treatment of heavy hydrocarbon stocks which contain asphaltic materials wherein the feedstock is subjected to visbreaking and deasphalting prior to catalytic hydrogenation, thus sacrificing asphaltene hydroconversion. The Beuther patent does not teach or suggest deasphalt treatment of the catalytic hydrogenation effluent.
The general object of this invention is to obtain lower boiling hydrocarbons from heavy hydrocarbon streams containing asphaltenes which are readily converted under mild hydrocracking conditions while employing a second more severe hydrotreatment to convert heavier materials without asphaltenes present, thus avoiding plugging and asphaltene precipitation problems in the hydrotreated effluent. An additional object of this invention is to treat heavy hydrocarbon streams containing asphaltenes in a manner which will remove the most difficultly converted asphaltenes from a mildly hydrotreated stream in which such asphaltenes are readily separated.
We have found that the objectives of this invention can be attained in a process which comprises hydrocracking the heavy hydrocarbon feedstock containing asphaltenes in a first hydrocracking zone followed by removal of the asphaltenes from the liquid effluent of said first hydrocracking step to form a deasphaltened liquid, and hydrocracking said deasphaltened liquid in a second hydrocracking zone.